Chris Brown's friend, Lowell Grissom Jr., who's been accused of raping a woman during a party at the singer's house ... is now officially on the L.A. County District Attorney's radar.

Law enforcement sources tell TMZ ... LAPD has sent the case to the D.A.'s Office. You'll recall, an anonymous woman claims Grissom, Brown and another woman held her captive in a room at Chris' house ... and that Grissom raped her twice.